If you save a Trimap, using the File menu’s Save As…, before actually computing the mask, the program confirms the save, but never actually saves it.
This is true when using Mask AI as standalone, or as a Photoshop plugin.
After computing the mask, things work fine (except for the kludgy dialog).
I routinely save the Trimap. Using a saved Trimap, it’s possible to go back and tweak a mask.